1. Field of the Invention
The present invention relates to a method for forming an insulating film suitably used as a gate insulator for an FET (Field Effect Transistor) such as a MOS (Metal Oxide Semiconductor) transistor embedded in an integrated circuit or as a dielectric film for a capacitor and to a method for manufacturing the integrated circuit using the above method for forming the insulating film.
2. Description of the Related Art
A thickness of the gate insulator for the MOS transistor to be embedded in the integrated circuit, in order to provide a predetermined electrical property to the MOS transistor, has to be controlled precisely.
It is possible to form a silicon dioxide film having a desired thickness comparatively precisely by using a method disclosed in Japanese Patent Application Laid-open No. Hei 11-283975 in which a surface of a silicon substrate is exposed to ozone and an environmental temperature is suitably controlled.
However, even if the silicon dioxide film having the desired thickness is formed on the silicon substrate, when the silicon dioxide film formed on the silicon substrate is exposed to an atmosphere, due to spontaneous oxidation of the silicon substrate, the silicon dioxide (SiO2) film continues growth. When it is left in the atmosphere for five hours, an amount of an increase in a thickness of the silicon dioxide film is up to 0.1 nm to 0.5 nm.
Therefore, when a thickness of a gate oxide film to be used for the MOS transistor has to be, for example, 2 nm, by employing the above conventional method, the silicon dioxide film having the precise thickness of 2 nm can be formed, however, the thickness of the gate oxide film composed of the silicon dioxide film is larger by 5% to 25% than the originally desired thickness. As a result, the growth of the gate oxide film caused by the spontaneous oxidation greatly changes the electrical properties of the MOS transistor. Moreover, since the gate oxide film having such the increased thickness occurs in the region of a semiconductor wafer in a non-uniform manner, when many MOS transistors are formed collectively on the semiconductor wafer, a great variation in the electrical properties occurs in each of the MOS transistors.
In view of the above, it is an object of the present invention to provide a method for forming an insulating film which is capable of inhibiting a spontaneous growth of a silicon dioxide film formed on a silicon substrate and of inhibiting an increase in thickness of the silicon dioxide film caused by exposure to an atmosphere.
It is another object of the present invention to provide a method for manufacturing an integrated circuit which is capable of preventing a variation in electrical properties caused by dispersion in thickness of the insulating film by controlling the dispersion in the thickness of the insulating film.
According to a first aspect of the present invention, there is provided a method for forming an insulating film made up of silicon dioxide on a silicon crystal including:
a step of allowing the silicon dioxide having a predetermined thickness to grow on the silicon crystal; and
a step of exposing a surface of the silicon dioxide to organic gas containing no hydroxyl group.
In the above aspect, it is desirable that low-molecular organic gas is used as the organic gas containing no hydroxyl group, which includes typically methane gas and ethane gas which are a paraffin saturated hydrocarbon. Moreover, unsaturated hydrocarbon such as ethylene or propylene may be used also as the organic gas.
In the foregoing, a preferable mode is one wherein the organic gas is paraffin low-molecular gas.
Also, a preferable mode is one wherein a growth of the silicon dioxide occurs by exposing the silicon dioxide to an atmosphere of ozone and, after the silicon dioxide has grown until it has a predetermined thickness, the ozone in the atmosphere is replaced with nitrogen gas or inert gas and then, organic gas containing no hydroxyl group is introduced into the atmosphere, in order to expose the surface of the silicon dioxide to the organic gas.
In the above aspect, to prevent reaction of the ozone with the ammonia gas, it is preferable that, after the silicon dioxide has grown until it has the predetermined thickness, prior to the introduction of the ammonia gas into the atmosphere, the ozone in the atmosphere is replaced with the nitrogen gas or the inert gas.
According to a second aspect of the present invention, there is provided a method for forming an insulating film made up of silicon dioxide on a silicon crystal, the method including:
a step of allowing the silicon dioxide having a predetermined thickness on a surface of the silicon crystal; and
a step of terminating binding of a dangling bond of a silicon atom on the surface of the silicon dioxide by causing a nitrogen atom to bind to the dangling bond of the silicon atom.
In the above aspect, when the surface of the silicon dioxide is exposed to the organic gas, the nitrogen atom in the organic gas binds to the gangling bond of the silicon atom to terminate the growth of the silicon dioxide or the organic gas component is adsorbed on the surface of the silicon dioxide layer by van der Waals force. Since the organic gas effectively inhibits permeation of an oxygen molecule, the growth of the silicon dioxide caused by spontaneous oxidation can be efficiently prevented.
In the foregoing, a preferable mode is one wherein a growth of the silicon dioxide occurs by exposing a crystal face of the silicon dioxide to an atmosphere of ozone and, after the silicon dioxide has grown until it has a predetermined thickness, the ozone in the atmosphere is replaced with nitrogen gas or inert gas, and then ammonia gas is introduced into the atmosphere in order to cause a dangling bond of a silicon atom on a surface of the silicon dioxide to bind to a nitrogen atom.
According to a third aspect of the present invention, there is provided a method for manufacturing an integrated circuit including:
a process of forming an insulating film made up of silicon dioxide on a silicon crystal, which includes a step of allowing silicon dioxide having a predetermined thickness to grow on a surface of the silicon crystal by exposing a crystal face of the silicon dioxide to an atmosphere of ozone, a step of replacing the ozone in the atmosphere with nitrogen gas or inert gas after the silicon dioxide has grown until it has a predetermined thickness, and a step of introducing organic gas containing no hydroxyl group into the atmosphere in order to expose a surface of the silicon dioxide to the organic gas.
In the foregoing, a preferable mode is one wherein the insulating film is a gate insulator of an field effect transistor or a dielectric film of a capacitor.
According to a fourth aspect of the present invention, there is provided a method for manufacturing an integrated circuit including:
a process of forming an insulating film made up of silicon dioxide on a silicon crystal, which includes a step of allowing the silicon dioxide having a predetermined thickness to grow on a surface of the silicon crystal by exposing a crystal face of the silicon dioxide to an atmosphere of ozone, a step of replacing the ozone in the atmosphere with nitrogen gas or inert gas after the silicon dioxide has grown until it has a predetermined thickness, and a step of introducing ammonia gas into the atmosphere in order to cause a dangling bond of a silicon atom on a surface of the silicon dioxide to bind to a nitrogen atom.
In the above aspect, the nitrogen atom to terminate the binding of the silicon atom on the surface of the silicon dioxide effectively inhibits the permeation of the oxygen molecule and therefore the growth of the silicon dioxide can be efficiently prevented.
In the foregoing, a preferable mode is one wherein the insulating film is a gate insulator of an field effect transistor or a dielectric film of a capacitor.
With the above configurations, since the permeation of an oxygen molecule into silicon existing under the silicon dioxide can be inhibited by exposing the surface of the silicon dioxide to organic gas containing no hydroxyl group, the increase in film thickness caused by spontaneous oxidation of the silicon dioxide can be prevented, thus enabling the thickness of the film to be maintained at a proper level. Moreover, the permeation of the oxygen molecule into silicon existing under the silicon dioxide can be prevented by terminating the binding of the dangling bond of the silicon atom on the surface of the silicon dioxide by nitrogen atoms and therefore the increase in film thickness caused by spontaneous oxidation of the silicon dioxide can be avoided, thus enabling the thickness of the film to be maintained at a proper level. Furthermore, by applying the method for forming the insulating film to formation of gate oxide films of MOS transistors in integrated circuits or to dielectric films of capacitors, manufacturing of the integrated circuit exhibiting a homogenous property having no variations in electronic characteristics is made possible.